This invention relates in general to a device for real-time thickness measurement during machining and, more specifically, to an improved ultrasonic thickness measuring assembly for such use.
Large integrally stiffened skins machined in one piece from an aluminum billet are gaining widespread use in space vehicles and high-speed aircraft designs. These structures must combine high strength with light weight. Weight penalties are particularly severe in space vehicle applications; in some cases thousands of dollars per pound. Dimensional accuracies on the order of .+-.0.005 inch which are commonly required exceed the capability of many milling machines and leave very little allowance for measurement error. Typically, all of a 2.5 inch billet except for a 0.050 skin thickness is milled away, removing some stresses and leaving others. This creates the potential for skin warpage, producing gaps between the machine bed and the face of the billet being machined. As the milling cutter passes over these gaps, undetected thinning of the skin can require scrapping of the entire part.
Further, compensation for machine backlash, tool wear, etc. is not possible. The necessary result is the use of broader tolerances, resulting in greater weight, plus increased waste where the machine goes out of even the broader tolerances.
An attempt has been made to provide real-time thickness measurement by mounting an ultrasonic transducer within the tool. Such a device is disclosed in U.S. Pat. No. 3,483,795 to Wranosky. While this device has several advantages, it has not been found to be fully effective for several reasons. Vibration from the tool spindle and tool, within which the ultrasonic transducers is mounted, make detection of the ultrasonic signal difficult, and at times impossible.
Thus, there is a continuing need for improved means for mounting ultrasonic thickness measuring transducers within machine tools.